Sky's Up July-September 2017 - Page 44

Things to do with your Earth-Moon model: Activity No. 1: Have an eclipse! 1. For this activity, you will need your Earth-Moon model, and another marble glued to the eraser end of a pencil to represent the Sun. We use a marble for both the Moon and Sun because the two objects appear to be the same size in our sky. 2. The person who holds the Earth is the observer. The person who holds the Sun stands 6 feet beyond the Moon and tries to hold the Sun model as still as possible. 3. The person who holds the Moon tries to move the moon-marble smoothly so that it passes exactly in front of the Sun allowing the Earth observer to see an “eclipse.” The Earth observer is allowed to give directions — “a little higher”, “too low”, etc. 4. Let groups of three try this activity. How many tries will it take your team to make an eclipse where the Moon model completely blocks out the Sun model? What do we learn? • The Moon’s orbit is tilted a bit (just 5 degrees.) This small tilt means that the Moon is usually a little above or below the Sun as seen from here on Earth – that makes eclipses very rare! • The Moon’s orbit carries it in front of the Sun, blocking out the light during an eclipse. Many people think that the Sun moves behind the Moon. In fact, this is not true. It is the Moon’s orbital motion that carries it in front of the Sun during an eclipse. • An eclipse is very brief. Because the Moon and Sun both appear quite small from Earth (about ½ degree wide) the eclipse does not last very long. The Sun is blocked for just 2-3 minutes during most eclipses! Activity No. 2 The Moon’s Orbit in Motion 1. On a paved area outside, have one Math it up! 1. The Moon’s diameter is 3,475 km. The Sun is 1,390,000 km – about 400 times larger. How can the Sun and the Moon appear to be the same size in our sky? 2. The Moon is about 385,000 km away from the Earth (this is the orbital radius). What is the circumference of the Moon’s orbit? 3. If the Moon orbits the Earth in about 28 days, what is its average speed in orbit? See the solutions at the bottom of the